A data center often includes servers, power supply equipment, lights, office equipment, miscellaneous equipment, and cooling equipment (collectively equipment components). Power supply equipment may include uninterruptible power supply (UPS) equipment, batteries, and power generators. Cooling equipment may include cooling towers, chillers, computer room air conditioners (CRAC), and the like. In general, a data center is designed to provide some number of kilowatts for the critical load, i.e., the electrically powered critical equipment can consume no more than the provided power.
Typically data center users want to ensure their facilities are as energy efficient as is financially sensible. The industry accepted metric used for quantifying efficiency is Power Usage Effectiveness (PUE). PUE is defined as Total Facility Power/IT Equipment Power. As such, a value approaching 1.0 would indicate 100% efficiency. Another metric used is Data Center Infrastructure Efficiency (DCiE) which is the reciprocal of PUE (1/PUE). A 0.33 DCiE would indicate that IT equipment consumes one third of a data center's power usage and would correspond to a 3 PUE. Other efficiency metrics exist, such as Data Center Performance Efficiency (DCPE) which is defined as useful work divided by total facility power.